Obesity is a major cause and contributor to health problems such as type II diabetes, coronary heart disease, increased incidence of certain forms of cancer and respiratory complications. It is a disease that is increasing at an alarming rate due to increased availability of high-fat diets, genetic susceptibility and a more sedentary way of life in modern society. Obesity can be defined as weight gain resulting from a mismatch of energy intake and energy expenditure. Food intake and energy metabolism are regulated, in part, by the interaction of neuropeptides and their receptors. Recently, the role that the hormone leptin plays in controlling appetite has been elucidated.
Leptin is a peptide hormone produced by fat cells, regulating both food intake and and metabolism by acting on leptin receptors in the hypothalamus. Increased fat stores leads to increased secretion of leptin, resulting in a signal to the hypothalamus to decrease food intake, whereas decreases in adiposity result in lower leptin levels and a stimulation of food intake. Melanin-concentrating hormone (MCH) has been identified as an orexigenic peptide that counterbalances the activity of leptin.
MCH is a cyclic 19 amino acid neuropeptide expressed in the zona incerta and lateral hypothalamus in response to both energy restriction and leptin deficiency. MCH is known to stimulate feeding when injected into the lateral ventricle of rats and the mRNA for MCH is upregulated in the hypothalamus of genetically obese mice (ob/ob) and in fasted control and ob/ob animals. Mice lacking MCH are hypophagic and lean with increased metabolic rate, whereas animals over-expressing MCH gain excess weight on both standard and high fat diets. MCH is thought to have effects on other nervous system functions as well (Nahon J L., The melanin-concentrating hormone: from the peptide to the gene. Crit Rev Neurobiol 8:221–262, 1994). An orphan G-protein coupled receptor (GPCR) was recently identified as a receptor for MCH.
Although there exists current pharmacologic therapies used to treat obesity, none of the current therapies achieve the U.S. Food and Drug Administration criteria for benefit measured by a 5% difference in mean weight loss, as weight loss efficacy is diminished by reduction of patient adherence to pharmacological therapy due to side effects of the drugs. Some of the side effects associated with current therapies include increased heart rate and blood pressure and uncontrolled excretion of fat in stools. Thus, there exists a medical need for agents capable of preventing or treating eating disorders, weight gain and obesity, that at the same time, have improved efficacy and safety.